1. Field of the Invention
The present invention relates generally to a system for charging electric vehicles, and particularly to a safe electric vehicle supply station with fault protection features.
2. Technical Background
Electric vehicles are becoming increasingly popular due to the rising cost of petroleum, the air pollution related to the use of petroleum based fuels, and the growing awareness that the supply of oil and gas is limited. In fact, there are some projections that indicate that the supply of oil and gas could be depleted in the near term. The advantages, therefore, of an electric vehicle over a conventional gas-powered vehicle are plainly recognized. On the other hand, there are difficulties associated with the electric vehicle that must be overcome before they become an efficient and effective replacement for conventional gas-powered vehicles. For example, conventional gas-powered vehicle are easily refueled within a few minutes. Electric vehicles typically employ batteries that may take hours to recharge. What is needed, therefore, is an infrastructure that includes readily available electric vehicle charging stations that may be accessed when the vehicle is at a parking location, e.g., at home, work, shopping, or at other such locations.
Electric vehicle (EV) charging stations may be configured to conform to a variety of form factors resembling known devices such as parking meters, filling station dispensers, residential charging units, etc. In each of these contemplated embodiments, the EV charging station provides the EV with electricity via an existing electrical distribution system. Accordingly, what is needed is an EV charging station that includes protective circuitry that prevents a user from experiencing shock or electrocution in the event of a fault condition.
The term electrical distribution system refers to the system employed to deliver AC power to a user from an AC power source (e.g., a utility company access point). An electrical distribution system typically includes at least one phase (hot) wire, a neutral wire and a ground wire. For example, a 120 VAC electrical distribution system typically includes a phase wire, a neutral wire, and a ground wire. A distribution system may also be configured as 240 VAC and 277 VAC systems. A 240 VAC system includes two phase (hot) conductors. Electrical distribution systems may include up to three phase conductors. In this case, the electrical power signals propagating on the conductors are 120° out of phase with respect to the signals propagating on the other phase conductors. From a mechanical perspective, the electrical distribution system may be implemented using a cable that bundles the wires together within exterior sheathing. The wires may also be disposed in rigid or flexible conduits. The ground conductor is connected to earth ground at the origin of the electrical distribution system, typically near an electrical distribution panel. The neutral wire, if provided, is very often also connected to earth ground at this point.
As an initial point, the terms “upstream” and “downstream” are defined as follows. Progressing through a branch circuit in a direction from the main breaker panel to a load is referred to as the “downstream” direction. The opposite direction, i.e., from a point in a branch circuit toward the main breaker panel, is referred to as the “upstream” direction.
The ground wire is an important safety feature in the system. The phase and neutral wires provide electrical power to motors, circuitry, lighting and various appliance loads. The ground wire and grounded portions of an appliance, on the other hand, are electrically isolated from the phase and neutral conductors such that little or no current normally flows in the ground wire. The grounding circuit is implemented by electrically connecting the ground wire to the appliance's metallic sheathing (if provided) or to one or more conductive surfaces within the appliance. The grounding circuit keeps the metallic appliance enclosure, or other such connected portions of the appliance, at or near ground potential to prevent the user from being electrocuted or from receiving an electrical shock. The grounding circuit comes into play in several scenarios. For example, the phase circuit wire insulation may become damaged such that the phase conductor contacts the grounded circuit. As another example, a miswiring condition may occur such that the phase conductor is improperly connected to the ground wire. In each of these cases, the ground path safely directs the resulting and potentially harmful currents to ground to eliminate any shock hazard.
The grounding circuit may also be safeguarded by the use of various types of protective devices such as over-current devices, ground fault circuit interrupters (GFCIs), arc fault circuit interrupters (AFCIs) and combination GFCIs/AFCIs. These devices may be disposed in the electrical distribution system at various locations. For example, protective devices may be employed in the distribution panel, electrical receptacles, wall outlets, portable housings, power interconnect devices, or electrical plugs.
An overcurrent device is responsive to the amount of phase current propagating in the circuit. This type of device limits the amount of current (amperage) being directed to a load, or multiple loads in combination. Many overcurrent devices are typically configured to interrupt a current that is greater than 15 Amperes. Other devices may have a 20 Ampere rating. Of course, overcurrent devices are configured to detect currents propagating to ground.
AFCIs have a different function. Unlike overcurrent devices, AFCIs are adept at detecting intermittent currents that arise from sputtering arc fault conditions. An arc fault is a discharge of electricity between two or more conductors and may be caused by damaged insulation on a hot line conductor or a neutral line conductor, or on both a hot line conductor and a neutral line conductor. The damaged insulation may cause a low power arc between the two conductors and a fire may result. An arc fault usually manifests itself as a high frequency current signal characterized by a particular signature. An arc fault signal typically includes a concentration of energy in certain frequency bands. Thus, an arc fault circuit interrupter (AFCI) protects the electric circuit in the event of an arc fault. Accordingly, an AFCI may be configured to detect various high frequency signals, i.e., the signature, and de-energize the electrical circuit in response thereto.
A ground fault is a condition that occurs when a current carrying (hot) conductor contacts ground to create an unintended current path. The unintended current path represents an electrical shock hazard. A ground fault may also result in fire. A ground fault may occur for several reasons. If the wiring insulation within a load circuit becomes damaged, the hot conductor may contact ground, creating a shock hazard for a user. A ground fault may also occur when equipment comes in contact with water. A ground fault may also be caused by damaged insulation within the electrical distribution system. A ground fault circuit interrupter (GFCI) is specifically designed to detect currents to ground. GFCIs differ from overcurrent devices because they detect much smaller currents. GFCIs are typically rated to interrupt leakages to ground that are greater than 6 mA. A charging circuit interrupting device (CCID) is a GFCI that trips at a predetermined threshold, 20 mA being typical. There are other devices, e.g., Ground Fault Equipment Protectors (GFEPs), which are configured to interrupt the circuit (to remove the fault) when the ground fault is greater than 30 mA. All of these devices serve to interrupt fault currents propagating in the ground circuit before the ground wire becomes overheated, or is compromised by an open circuit condition. An open circuit in the ground path, therefore, represents an electrical shock hazard because the current flowing in the ground wire cannot flow to ground when the ground wire is broken. Instead, the current will seek the best available path to ground and, unfortunately, the best available path to ground might include a human being. Accordingly, a device commonly referred to as a ground continuity monitor (GCM) may be employed to determine if the ground conductor is intact.
In one approach that has been considered, a GCM is configured to provide an indication means that communicates to the user whether the circuit is grounded or not. One drawback to the considered approach is that the GCM allows power to be provided to a load in the presence of a miswired condition or absence of a ground conductor.
In another approach that has been considered, GCM circuitry may be connected to the hot, neutral and ground conductors for various reasons. One drawback to this approach is that the interconnected circuitry may provide a leakage path. As noted above, conductive portions of appliances (such as metallic outer sheathing) are typically connected to the ground prong of the corded plug set to provide a path to ground. If the ground prong of a corded appliance is inserted into the ground terminal of a receptacle and the ground conductor is not present, or is compromised in some way, a leakage current is able to flow through a person contacting the conductive portions of the appliance. Of course, this represents a shock hazard. Clearly, overcurrent issues, ground fault and/or ground continuity issues must be taken into account in EV charging stations.
There is an additional layer of complexity when one considers that each branch circuit in an electrical distribution system may be protected by a plurality of protective devices (e.g., overcurrent devices, GFCIs, AFCIs, etc.). When multiple protective devices are connected in series, it is preferable that the device furthest downstream from the main breaker panel perform the protective task. With respect to overcurrent devices, each branch circuit in the electrical distribution system is commonly protected by a plurality of overcurrent devices (e.g., fuses, breakers, etc.). For example, a main breaker panel includes a main breaker that may be employed to interrupt the main AC power source provided by the utility company. The breaker panel, of course, divides the AC power source into a plurality of branch circuits. Each branch circuit is provided with its own individual circuit breaker or fuse. The branch circuit breaker is also located in the main breaker panel. In some applications, a branch circuit may be configured to provide power to a remotely located subpanel. In this case, the circuit splits into sub-circuits at the sub-panel by way of additional circuit breakers. In any event, the branch circuit provides power to wiring devices, power taps, power strips, appliances and dedicated loads. Some of these devices may include additional overcurrent capability. Similar issues arise with GFCIs in that they are also commonly located throughout the electrical distribution system in panels, wiring device wall boxes, attachment plugs, and appliance cords. Accordingly, there may be circuits that include multiple GFCIs disposed in series. In the event of a fault condition, the GFCIs could be detecting the same ground fault condition.
One reason for the strategy of using the device furthest downstream is that it is usually the one closest to the user. Resetting a device in the room where the user is located in preferable to walking down to the basement where the main panel is typically located. Furthermore, the downstream device limits power interruption to a relatively smaller portion of the branch circuit where the fault hazard is located. This means that only a small part of the circuit is interrupted; AC power continues to be provided to the majority of electrical distribution system.
In one approach that has been considered for implementing this strategy, the time-current curves of the devices in the circuit are coordinated. Regarding overcurrent devices, the main breaker is chosen to have a higher trip threshold (I1) when compared to the downstream circuit breakers (I2), i.e., I1>I2. The downstream device is the only one that responds to overcurrents less than (I1) Amperes. Thus, the downstream device would have a faster interruption time than the upstream device. The downstream device could also interrupt certain overcurrents that are greater than (I1) Amperes. A downstream ground fault device is usually implemented using a GFCI that trips at 6 mA. Thus, the downstream GFCI trips when the differential current is between 6 mA and the trip threshold of the upstream device and the upstream device would not trip. The 6 mA GFCI device may be chosen to have faster interruption times than the upstream device, in which case, the GFCI could also interrupt some currents that are greater than the trip threshold of the upstream device before the upstream device had the ability to react. An upstream GFEP device would be selected, therefore, to have a comparatively greater differential current interruption threshold, typically a value within the range of about 30 mA to several amperes. This device is intended to interrupt ground faults that have high energy; if faults of this type are allowed to persist, they would cause damage to equipment or wiring, and could lead to a fire.
Proposed EV charging stations must be equipped with attachment wires that are terminated to the branch circuit wiring inside a junction box. Alternatively, the charging station may include an attachment plug that plugs into an electrical receptacle disposed in the branch circuit. These have been referred to as “traveler sets.” At the same time, electric vehicles (EVs) include a battery assembly that is configured to provide power to the vehicle's drive train. When the stored energy in the battery is depleted, the battery must be recharged. Thus, the EV may be equipped with a power cord that is coupled to the battery assembly inside the vehicle. The other end of the power cord includes a user accessible attachment plug that mates with the EV recharging station, and, by extension, to the electrical distribution system. Like traditional vehicles powered by internal combustion engines, an EV is enclosed and shaped by a metallic body that is connected to a metallic frame. The EV frame and body are substantially isolated from ground by rubber tires. Should there be a discontinuity in the ground circuit, the risk of shock or electrocution from contacting an energized EV body or frame is considerably greater than the risks associated with an ordinary appliance by virtue of the extensive surface area of the EV body.
In light of the ground continuity issues, ground fault and arc fault issues, and the presence of multiple protective devices in a given circuit, the introduction of electrical vehicles into the market will place new safety demands on the traditional electrical distribution system. What is needed, therefore, are EV charging stations that are configured to safely provide electricity to electrical vehicles from existing electrical distribution systems.
For example, it should be expected that EV users will attempt to repair or replace the EV's power cord or attachment plug because of the wear and tear on the power cord after repeated usage. In the process, it is possible to transpose the neutral and ground conductors or transpose the phase and ground conductors. Thus, the grounding path from an EV charging station to the electrical distribution system must be reliable.
Another issue relates to protecting the EV user from the risk of shock during the recharging cycle. One must keep in mind that the EV body represents a very large conductive surface that is in contact with both the driver and passengers on a daily basis. Further, the body is electrically isolated from ground by the vehicle's tires. The EV, as well as the high frequency circuitry associated with the on-board battery charging circuit, is constantly being exposed to mechanical shock, vibration, moisture and dirt. These factors may increase the risk of discontinuity in the ground connection. On the other hand, during normal operation of the electric vehicle there is high frequency noise generated by the switch mode power supply, and currents may be directed to ground via electronic noise filters or other such circuitry. Accordingly, Underwriters Laboratories has established a new category of personnel protection equipment known as the charging circuit interrupting device (CCID) whose trip threshold is 20 mA, if a ground circuit is verified to be present (by a ground continuity monitor). Even though these currents are safe, they will cause an ordinary 6 mA GFCI to nuisance trip, i.e., an interruption of the charging cycle for non-fault related reasons. UL has included other stipulations in the construction of the charging station and the electric vehicle to compensate for what would otherwise represent a loss in safety.
One of the drawbacks associated with the CCID relates to the aforementioned fact that when multiple protective devices are connected in series, it is preferable that the furthest downstream device perform the protective task. The furthest downstream GFCIs are rated at 6 mA, and are disposed upstream of the EV charging station. This represents an inherent mismatch for an EV charging station protected at 20 mA. To be clear, one should understand that the National Electrical Code (NEC) has established safety requirements for the electrical distribution system that cannot be ignored. The NEC has a requirement that at least some receptacles in the electrical distribution system be provided with GFCI protection. Thus, the presence of EV charging station violates the established device coordination protocol of placing less sensitive protective devices upstream of the relatively sensitive 6 mA GFCIs. Because these GFCIs are upstream of the EV charging station they would have a propensity to nuisance trip. There is also the aforementioned inconvenience of having to walk down to the basement to reset the tripped device before recommencing the charging cycle. Obviously for this type of scenario the traditional coordination of devices is not an option.
What is needed is a protected EV charging station, or a protection device for use with an EV charging station that addresses the needs described above. An EV protection device is needed, whether it is incorporated into the EV charging station or used in conjunction with it, which is configured to interrupt a fault condition proximate the downstream charging stating before an upstream protective device trips. In doing so, it must have a predetermined trip threshold above the comparatively lower trip threshold of an upstream protective device. An EV protection device is needed to selectively prevent an upstream protective device from being able to detect a predetermined fault condition in a branch circuit if the predetermined fault condition is in a load downstream of the downstream protective device. Alternatively, an EV protection device is needed such that it is configured to allow an upstream device to trip in response to a fault condition in a load that is not parallel to the downstream device. EV charging stations are also subject to end of life conditions in which the failure of a component or system impairs the protective function. What is also needed, therefore, is a protected EV charging station that does not provide unprotected power when an end of life condition occurs.